If you're doing everything right but still experiencing weight gain, a medical problem could be to blame. Here, five frequently missed causes of excess pounds.

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If the needle on your bathroom scale starts creeping upward, or refuses to head downward, you'll probably suspect the cause of this random weight gain is too many doughnuts, not diseases or drugs. But while the usual culprits-too much food, too little exercise-do account for most excess poundage, there are some surprisingly common medical conditions and widely used prescriptions that can add anywhere from a little to a lot-a whole lot-of excess weight. Here's a checklist of what to watch for if you inexplicably find weight either packing on or unwilling to pack up and go.

1. Hormonal Havoc

You'd think 40 or more extra pounds would be a clue that something's amiss. Yet many of the 7-10 percent of premenopausal women with polycystic ovary syndrome (PCOS) often go for years unaware that their weight gain is in part due to this underdiagnosed condition, in which the ovaries and sometimes the adrenal glands, for unknown reasons, pump out too much testosterone, according to Andrea Dunaif, M.D., chief of endocrinology at Northwestern Memorial Hospital and professor of metabolism and molecular medicine at Northwestern University Medical School. Because the pounds typically pile on gradually beginning around puberty, or sometimes don't surface until post-pregnancy weight refuses to budge, it's frequently not obvious to PCOS sufferers, or their doctors, that there's a medical trigger. Possible tip-offs of PCOS include thinning hair, excess facial hair, severe acne, irregular periods, impaired fertility-all hallmarks of a hormal imbalance.

It's not the extra testosterone that triggers the weight gain, though. So what does? Short answer: Nobody knows, Dunaif says. While there seems to be a genetic component to PCOS-it runs in families-and a genetic component to the associated weight gain, there's little to explain why some of those diagnosed develop weight problems while others do not. It is clear that cultural and environmental factors play a part because Europeans, and Americans on the coasts, who may feel more social pressure to be skinny, gain much less weight on average than do their (sometimes literal) sisters in middle America. The encouraging side of this is that while many women with PCOS feel like their weight is an immovable number (and treatment for PCOS does not help with weight loss), studies show that almost any woman with PCOS, treated or not, can, if put on a supervised diet and exercise program, lose 10 percent or more of body weight, Dunaif notes. Dropping such a moderate amount of weight often will, in turn, push male hormone levels down, leading to a resumption of regular periods and improved chances of conception.

2. Thyroid Thickness

For the most part, blaming a sluggish thyroid for excess weight falls in the "you wish" category. "A lot of overweight people sort of hope they have hypothyroidism because it's treatable," comments Howard Eisensen, M.D., director of Duke University's Diet and Fitness Center. "But it's rare to find someone who's significantly overweight because of an underactive thyroid. Even if there is decreased thyroid function, correcting it doesn't do much to correct overweight because it doesn't cause much gain to begin with." If weight creep is on a small scale-in the 5- to 10-pound ballpark-it's possible that hypothyroidism is behind it, though. If you have other telltale symptoms, such as brittle hair and nails, dry skin and a tendency to feel cold, definitely get checked out, or perform a thryroid self-exam. If your thyroid is to blame, treatment should shrink you a bit, but not because of much fat loss.

Another name for hypothyroidism is "myxedema," which describes a kind of swelling from thick fluidlike tissue that is a hallmark of chronic low thyroid, explains George Bray, M.D., Boyd Professor at Louisiana State University. Most of thyroid-prompted weight gain, therefore, is actually due to excess fluid, not fat; correcting the thyroid problem banishes soggy tissue, along with its poundage, pretty effectively.

3. The Weight of Water

As Bray points out, extra pounds don't always equal fat, but are sometimes due to fluid retention-familiar to most women from premenstrual symptoms. If puffiness isn't related to the menstrual cycle, though, it shouldn't be ignored. "If someone's retaining a lot of water-enough to add more than a couple of pounds-they'd better get to their physician very quickly to make sure they don't have heart or kidney failure, both of which can cause edema, or swelling," cautions Robert Berkowitz, M.D., medical director of the University of Pennsylvania Weight and Eating Disorders Program, though he adds that such problems are much more likely to afflict older women. "If you push a fingertip into your skin and it leaves a real indentation rather than springing back, that's a tip-off that it's fluid, not fat." Other symptoms include shortness of breath (congestive heart disease), decreased urine output and loss of appetite (kidney failure), and fatigue and increased abdominal girth even without weight gain, for both. Liver disease and certain cancers can cause abnormal fluid accumulation in the abdomen as well, so any big boost in your waist size, with or without weight gain, warrants a look by your doctor, Eisensen advises.

4. A Knot In Your Stomach

Unlikely, but worth mentioning: "If women have rapid, unexplained weight gain, it's possible, though rare, that they have a tumor," Eisensen reports. One example: ovarian tumors, some of which are benign, such as a dermoid tumor, a weird conglomeration of various body tissues (sometimes including teeth) that grow in the abdomen. "We've had patients gain over 100 pounds because of a huge ovarian tumor in their belly," he adds. Again, don't ignore any disproportionate expansion of your middle-check it with a physician.

5. An Rx That Rounds Your Weight Up

Do you take any medicines on a regular basis? Then there's a chance that one of them may be nudging your figure toward the fuller side. "It may be medications, more than diseases, that tend to contribute to weight problems," Eisensen says, "and it's helpful for people to know that there may be alternatives." Some medicines that commonly cause weight gain:

Anti-depressants: Of the widely prescribed SSRIs (selective serotonin reuptake inhibitors) and SNRIs (selective serotonin and norepinephrine reuptake inhibitors), many researchers and clinicians believe that paroxetine tends to produce the most weight gain, though typically not more than several pounds. Other antidepressants, like fluoxetine, sertraline and venlafaxine also may lead to weight gain, especially if used long term. "Short-term use of most SSRIs and SNRIs is not associated with weight gain," Eisensen says, "but if people are on them a year or longer, they can gain a few pounds or more."

One of the most popular mood stabilizers is bupropion, which if anything, often helps patients drop a few pounds (but side effects may not make it a good choice for all overweight people with depression, Eisensen says). The point is that within each class of anti-depressants are some that tend to produce more weight gain, and some that tend to produce less, Bray explains.

Anti-diabetes drugs: Ironically, medications for Type II diabetes-the kind caused primarily by obesity-are often responsible for further weight gain, creating a vicious cycle. Although it's not suitable for everyone, one effective anti-diabetes drug called Glucophage does not increase weight, says Berkowitz. And, he says, even some patients on other anti-diabetes drugs may be able to lose weight, under their doctor's supervision, by scaling back on their dosage or adding a weight-loss drug such as orlistat, which partially blocks fat absorption.

Oral contraceptives: But the low-dose pills commonly prescribed now won't add more than a few extra pounds, according to Berkowitz. Recent studies also back up that there is less of a connection between certain birth control and weight gain than previously thought.

Steroids: The most commonly prescribed are adrenocorticoids, used to control severe autoimmune problems, including asthma, arthritis, lupus and inflammatory bowel disease. Long-term use can increase appetite and hike weight by 20 pounds or more, Berkowitz says, but because the symptoms these steroids alleviate are potentially life-threatening, you don't have much choice but to be on them when you need to be. However, doctors should be vigilant about cycling patients off medication when they don't need it, which can help them lose some of the weight they have accumulated, he says.

"Many people who are on a medication and start to gain weight simply stop taking their medicine. Don't do that!" Berkowitz cautions. "Keep taking it while you ask your doctor about switching to something else." Whether a drug will cause you to gain weight is hard to predict, Eisensen says. "An anti-depressant that pushes one person's weight up will push another's down: Part of one person's problem may be unrestrained eating, and as the depression comes under control, so does the emotional eating," he says. "If a medication is a good choice for you, wait and see how it affects you, or look for another way to avoid the weight gain."

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